Rational Engineer

Introduction

During a process upset due to blocked outlet scenario, the fluid flow to the system may continue to add more fluid into the system. This would result in pressure build-up in system which is higher than its operating or even design pressure. If the equipment gets over-pressurized then it would lead to leaks or rupture from the equipment or piping. The hazardous and/or flammable process fluid would release in an uncontrolled manner, which is also termed as Loss of Process Containment (LOPC). Needless to say that this creates harm to people, assets and environment, which needs to prevented from occurring.  

In such cases two levels of protection [e.g., High pressure trip and Relief valve/HIPPS] are recommended in Process services as per guidelines such as per [1] or [2] . If you have come across any of the following queries, then you have come to the right place:

• How to specify the closing time of HIPPS valve?
• What is Process Safety Time (PST) and how to estimate it?
• How to estimate PST without use of dynamic simulation software? 
• How to specify the closing time of a Process shutdown/IPF valve using manual calculations?

In order for the barrier to be effective, the inlet shutdown valve should close fast enough to avoid pressure build-up exceeding the downstream system design limits. This article will explain the calculation methods, its assumptions/limitations and an excel spreadsheet to answer these questions.

Overpressure protection system by means of Instrumented Protective Function (IPF):

It is common practice to provide high pressure trip closing the inlet valve or source, which would form the primary barrier. In case of failure of this primary barrier, a secondary barrier by means of relief valve or a High Integrity Pressure Protection System (HIPPS) system would be provided. The purpose of both the barriers are to protect the under-rated system from overpressure and from LOPC. The two levels of protection are recommended in Process services as per guidelines such as per [1] or [2] .

The system is designed such that the highest accumulated pressure in the downstream under-rated system does not exceed the maximum Allowable Incidental Pressure (MAIP). The MAIP may be defined as 10% of the design pressure. Different codes and company guidelines such as [3] / [4] / [5]/ [6] / [7] provide specific definitions or requirements. As a conservative approach, the pressure build-up may be limited to design pressure of downstream under-rated system.

Response time of the IPF or HIPPS valve to close:

The total response for the IPF/HIPPS valve closure should be such that the flow which continues during the closing of the valve should not cause overpressure in the downstream under-rated system.

The total response time of the Safety Integrated Function (SIF) loop comprise of the following:

1. Response time of the initiators (pressure transmitters): Typically, in milliseconds but may be considered as 0.5 seconds
2. Response time of the logic solver (IPF/SIL system): Typically, in milliseconds but may be considered as 0.5 seconds
3. Response time of the final element (IPF or HIPPS valve) to go from full open to fully closed position: To be specified based on estimated/available time for pressure build-up from PZHH to RV set point. This is also called as available Process Safety Time (PST). The closing response time of the SIF loop (a+b+c) should be less than the available PST. Some companies specify the response time 20% lower than the available PST.

Manual Calculation of Pressure build-up based on incoming flow rate of gas and liquid:

Universal gas law equation can be applied to estimate the build-up of pressure in downstream under-rated system based on molar rate of incoming gas phase.

Equations:

Universal gas law:

System gas volume:

In case the incoming fluid also contains liquid in it, then the available vapour space/volume in the downstream system (for pressure build-up by gas moles) is reduced at every time step based on the incoming liquid rate.

Number of initial moles in the downstream low-pressure system:

Molar rate of incoming gas stream (kmoles/h):

System Pressure:

Assumptions:

1. It is assumed that incoming fluid rate remains same through-out the pressure build-up event and does not change or reduce. This simplifies the manual calculation process by considering the same incoming fluid rates at every time step.
2. No credit for reduced incoming rate considered due to closing of the IPF or HIPPS valve. These shutdown valves are typically ball valves with equal percentage valve characteristics. As the ball valve starts closing then after certain % of valve closure, the valve capacity would start to reduce. However, no credit of reduced valve capacity is considered.  
3. For the purpose of pressure build-up calculation, the pressure continues to build even after relief valve set pressure is reached since no credit for flow going out of the system is considered. This is because the intent of this exercise is to plot the pressure build-up VS time graph and estimate the available PST
4. Average fluid temperature and compressibility factor between initial and finals conditions are assumed to be same as that at initial conditions

Calculation using steady-state time steps:

Step-1: Take note of the incoming liquid volumetric rate, if applicable

Step-2: Calculate incoming molar rate of gas using the formula,

Step-3: Calculate total number of moles of gas in the under-rated system at initial conditions using the formulae,

Step-4: Calculate the number of moles added into the system at time step t=1 sec,

Step-5: Estimate the gas volume available in the system at time step t=1 sec,

Step-6: Estimate the system pressure at time step t=1 sec,

Step-7: Repeat the above steps 3 to 5 for subsequent time steps.

Step-8: Estimate Process Safety Time (PST) and HIPPS valve closure time

Identify the time take to reach following:

• Time taken to reach High pressure trip set point
• Time taken to reach Design pressure / Relief valve set pressure

Available Process Safety Time (PST),

Total response time of SIF loop (initiator + logic solver + final element),

e.g. Margin on PST = 1.2 times or 20%

Required total response time of SIF loop (a+b+c):              

1. Initiator (pressure transmitter) response time:  (say, ≤ 0.5 sec)
2. Logic solver (IPS) response time:  (say, ≤ 0.5 sec)
3. HIPPS/ IPF valve closure time: 

Excel spreadsheet to perform pressure build-up and PST estimation:

If you would like to have the already built excel spreadsheet to perform above calculations, then follow the instruction given below.

Do leave your comments on how you found this spreadsheet, where did you use it and did it work well to complete your task. We would love to hear your feedback.

References

[1] “API RP 14C - Analysis, Design, Installation, and Testing of Safety Systems for Offshore Production Facilities”.
[2]     “ISO10418 - Petroleum and natural gas industries — Offshore production installations — Process safety systems”.
[3]     “NAM guideline NDG 40-G-0-51 (April 1995) - HIPPS”.
[4]     “DEP 32.80.10.10 (Feb 2020) - Safety instrumented systems”.
[5]     “DIN 3381 (1984) - Safety devices for gas supply installations operating at working pressure up to 100 bar”.
[6]     “TOTAL GS EP SAF 260 (2005) - Design of High Integrity Protection Systems (HIPS)”.
[7]     “API STD 521, SEVENTH EDITION, JUNE 2020 - Pressure-relieving and Depressuring Systems”.